Dormancy and foliar density regulation in Thalassia testudinum

The hypothesis that the tropical seagrass, Thalassia testudinum from the Puerto Morelos reef lagoon (Mexican Caribbean) has a dormant apical meristem bank (as opposed to a dormant axillary meristem bank) was investigated by morphological and anatomical analyses of vertical shoot apices. Shoots were grouped as follows: (1) dead shoots (no foliar structures or active meristems), (2) developed shoots (bearing juvenile or mature foliar structures with active meristems), and (3) undeveloped shoots (bearing rudimentary foliar structures but with active meristems). In two beds, regressions of shoot density vs. proportional number of shoots in each of the above-mentioned categories, had negative slopes for developed shoots and positive slopes for undeveloped shoots, whereas no relationship was found between density and proportion of dead shoots, These data suggested that foliar shoot density was regulated by inhibition of foliar development, through suppression of meristem activity. Foliar shoot density increased significantly after experimental nutrient addition ana density of the developed shoots was 544.3 shoots per meter square for control plots and 1044.1 shoots per meter square for fertilised plots. Density of undeveloped decreased after fertilisation and densities were 623.9 and 194.2 shoots per meter square for control and fertilised plots, respectively, indicating that nutrient addition resulted in meristem re-activation in the undeveloped group. These results confirm the previous proposed existence of a dormant apical meristem banks for T. testudinum. (C) 2000 Elsevier Science B.V. All rights reserved.